I use Econ and it improves my mileage approx. 2-3mpg. consistently and I prefer it for drivability. Unless you, the driver, override what Econ does, it has to work.
I consistently get 33 to 34 in city driving and just returned from a thousand mile mixed interstate and two lane trip and the iMid showed exactly 40.0mpg. I just used Top Tier any brand gas of both regular and premium octanes depending on price at the time. Mileage usually boils down to driving habits, terrain, and traffic IMO. Do check what other's have advised though. FWIW.